FISHERY BULLETIN: VOL. 70, NO. 3 



. 



40 



"20 



25 30 10 15 



Mean Fork Length (cm) 



Figure 10. — Relationship between the size of single- 

 school summer purse-seine catches of Atlantic menhaden 

 and the mean length of fish in the catch sample, 1955-62. 



differences in size of summer schools, which are 

 independent of fish length, probably occur. 



NUMBER OF FISH WITHIN SUMMER 



SCHOOLS IN RELATION TO 



FISH LENGTH 



The number of fish within a school varied 

 widely but, on the average, decreased as fish 

 length increased. A plot of the estimated num- 

 ber of fish on the mean length of fish within the 

 275 single-school summer catches (Figure 11) 

 indicates an inverse relationship between the two 

 variables. The trend appears curvilinear; how- 

 ever, there is little difference among the corre- 

 lation coefficients between the numbers of fish 

 and their mean length (r = — 0.599) , between 

 log number and mean fish length (r = — 0.597), 

 or between log number and log mean fish length 



(r = — 0.585) ; all coefficients differ significantly 

 from 0. The estimated number of fish within 

 schools ranged from 500 (mean fish length 28.9 

 cm) to 655,400 (mean fish length 16.8 cm) , with 

 a mean of 86,776 fish. 



In interpreting the above findings, it must be 

 kept in mind that estimates of fish number are 

 indirect and therefore subject to systematic bias. 

 Furthermore, changes in the relation of school 

 size (weight) and fish length between years were 

 indicated in the previous section; therefore, a 

 common regression equation may not be repre- 

 sentative of actual changes in the numbers of 

 fish of given size within schools. Accordingly, 

 a trend line was not fitted to the data in Figure 

 11. 



700 



600 - 



500 - 



3 400 



0) 



E 



300 - 



200 - 



100 - 



10 



15 20 25 30 



Meon Fork Length (cm) 



35 



Figure 11. — Estimated number of Atlantic menhaden 

 in 275 single-school summer purse-seine catches plotted 

 against the mean length of fish in the catch sample, 

 1955-62. 



708 



